This invention concerns a widescreen extended definition television (EDTV) system wherein auxiliary baseband information modulates an auxiliary carrier signal.
A conventional television receiver, such as a receiver in accordance with NTSC broadcast standards adopted in the U.S. and elsewhere, has a 4:3 aspect ratio (the ratio of the width to the height of a displayed image). Recently, there has been interest in using higher aspect ratios for television receiver systems, such as 2:1, 16:9 or 5:3, since such higher aspect ratios more nearly approximate or equal the aspect ratio of the human eye than does the 4:3 aspect ratio of a conventional television receiver. Video information signals with a 5:3 aspect ratio have received particular attention since this ratio approximates that of motion picture film, and thus such signals can be transmitted and received without cropping the image information. However, widescreen television systems which simply transmit signals having an increased aspect ratio as compared to conventional systems are incompatible with conventional aspect ratio receivers. This makes widespread adoption of widescreen systems difficult.
It is therefore desirable to have a widescreen system which is compatible with conventional television receivers. It is even more desirable to have such a compatible widescreen system with provisions for enhancing or extending the definition of the displayed image so as to provide extra image detail. For example, such widescreen EDTV (extended definition television) system may include apparatus for providing a progressively scanned image. An example of one such widescreen EDTV system is disclosed by M. A. Isnardi, et al., in an article titled "Encoding for Compatibility and Recoverability in the ACTV System", published in IEEE Transactions on Broadcasting, Vol. BC-33, No. 4, December 1987. In that system a television signal including first, second, third and fourth components is developed. The second component (high frequency side panel information) and the third component (extra high frequency luminance detail information) are quadrature modulated on an auxiliary subcarrier. guadrature demodulation is used to recover the second and third components at a receiver.
In the case of conventional quadrature modulation, each of the upper and lower sidebands surrounding the quadrature carrier contains both of the first and second quadrature modulating signal components. Proper separation of quadrature modulating components requires that a reference oscillator, such as the local oscillator of a receiver's quadrature demodulator section, exhibit an accurate phase characteristic to maintain isolation between the two quadrature modulating signal components. An error in the phase of the local reference oscillator will produce crosstalk between the two demodulated components, which crosstalk may be noticeable if the phase error is large enough. If the phase error is large enough, for example 90.degree., the information content of the two components will interchange upon demodulation, resulting in serious image distortion. Undesirable crosstalk effects may also be produced as a result of signal "ghosts". In such case the quadrature carrier will appear at an arbitrary phase which may be sufficient to produce noticeable distortion of the information contained in the modulating components. The potential difficulties associated with the existence of a phase error in a quadrature modulation/demodulation scheme are avoided in a system in accordance with the present invention.